JUNE 2001 WANG AND ROGERS 1481 A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic. Part I: Cyclone Structure and Evolution CHUNG-CHIEH WANG* AND JEFFREY C. ROGERS Atmospheric Sciences Program and Department of Geography, The Ohio State University, Columbus, Ohio (Manuscript received 24 May 1999, in ®nal form 26 September 2000) ABSTRACT General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.58 resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland±Iceland region is higher than previously thought, and some of the events can be extremely violent. Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle. For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a ``parent cyclone'' with appreciable strength exists to the north/northeast of the incipient system. 1. Introduction 1 Ber 5 24 sin(f)/sin(608), (1) Rapid intensi®cation of extratropical cyclones, or ex- where f is the latitude of the cyclone center. Since the plosive cyclogenesis, has been an area of active mete- criterion is latitudinally adjusted to re¯ect the increase orological research during the past two decades due to in geostrophic wind speed, the required 24-h pressure onshore and offshore hazards and the failure of early drop for 1 Ber increases from 13.9 hPa at 308 to 27.7 operational models to predict it (Silberberg and Bosart hPa at the poles. The deepening rate in some extreme 1982; Sanders 1986b, 1987; Sanders and Auciello cases can well exceed 1 Ber, and two famous examples 1989). Sanders and Gyakum (1980) ®rst studied explo- are the Queen Elizabeth II storm in 1978 (Gyakum sive cyclones (or ``bombs'') and de®ned them as low 1983a; Kocin and Uccellini 1985) and the Presidents' systems whose central sea level pressure falls at an av- Day cyclone in 1979 (Bosart 1981). eraged rate of at least 1 Bergeron (or 1 Ber, in hPa Sanders and Gyakum (1980) produced the ®rst ex- day21), and plosive cyclogenesis climatology, updated by Roebber (1984), showing it to be primarily a cold season mar- itime phenomenon with highest frequencies over the northwestern Paci®c and northwestern Atlantic along * Current af®liation: Department of Environmental Management, major baroclinic zones. Although they suspect that both Jin-Wen Institute of Technology, Taipei, Taiwan. the frequency and intensity are likely underestimated over regions of sparse data coverage, studies by them and several others (e.g., Gyakum et al. 1989; Chen et Corresponding author address: Dr. Chung-Chieh Wang, Depart- ment of Environmental Management, Jin-Wen Institute of Technol- al. 1992) provide us with the best explosive cyclogenesis ogy, 99 An-Chung Road, Hsin-Tien City, Taipei, Taiwan. climatologies to date. E-mail: [email protected] Studies during the past two decades indicate that ex- q 2001 American Meteorological Society Unauthenticated | Downloaded 09/29/21 01:23 AM UTC 1482 MONTHLY WEATHER REVIEW VOLUME 129 plosive cyclones are like ordinary ones, fundamentally sive study is yet to be done. The purpose of this paper driven by baroclinic instability (Sanders 1986a; Man- is, therefore, to compare and contrast explosive cyclones obianco 1989b; Wash et al. 1992), with other factors in different sectors of the North Atlantic through a com- also contributing signi®cantly to the deepening rate. posite study using a large number of cases, with an These include 1) strong upper-level forcing (Rogers and emphasis on northeastern Atlantic. Bosart 1986; Macdonald and Reiter 1988; Hirschberg The present paper (Part I) employs a more qualitative and Fritsch 1991; Lupo et al. 1992), 2) intrusion of approach and focuses on cyclone structure and evolu- stratospheric high potential vorticity (PV) air (Bosart tion. Part II (in preparation) presents results of a vor- and Lin 1984; Uccellini et al. 1985; Zehnder and Keyser ticity budget analysis from a quantitative perspective. 1991; Reader and Moore 1995), 3) latent heat release Section 2 describes the data and methodology used to (Gall 1976; Anthes et al. 1983; Gyakum 1983b; Eman- identify, select, categorize, and composite our cases. uel et al. 1987; Mullen and Baumhefner 1988; Kuo et Composite results of key variables at various levels and al. 1991b; Whitaker and Davis 1994), 4) surface energy on vertical cross sections are presented in sections 3 and ¯uxes from the ocean (Atlas 1987; Davis and Emanuel 4, respectively. Section 5 provides further discussion, 1988; Fantini 1990; Nuss and Kamikawa 1990; Kuo et and section 6 summarizes the major ®ndings of this al. 1991a; Hedley and Yau 1991), and 5) enhanced local paper. baroclincity from differential diabatic heating (Bosart 1981; Rogers and Bosart 1991). Rapid development is the result of nonlinear interaction among all these pro- 2. Data and methodology cesses, although all factors need not be present (Uccel- a. ECMWF dataset lini 1990; Gyakum et al. 1995) and their importance could vary considerably in individual cases (e.g., Reed In this study, we use the European Centre for Me- et al. 1993). Nonetheless, since strong upper-level forc- dium-Range Weather Forecasts (ECMWF) operational ing exists both over land and ocean, it is quite clear that global analysis (Level III-b), archived at the National the effect of abundant moisture, supplied through ma- Center for Atmospheric Research from January 1985 to rine boundary layer processes, in lowering the stability March 1996. Data are twice daily (0000 and 1200 UTC) is what makes explosive cyclogenesis an almost exclu- on a regular 2.5832.58 latitude±longitude grid, and sive maritime phenomenon. In more recent years, high- variables include geopotential, temperature, and three- resolution datasets obtained through ®eld experiments dimensional u, y, and v (omega) wind components at (e.g., Dirks et al. 1988; Hadlock and Kreitzberg 1988) 14 pressure levels (1000, 850, 700, 500, 400, 300, 250, allow analysis of detailed mesoscale structure and evo- 200, 150, 100, 70, 50, 30, and 10 hPa), as well as relative lution (e.g., Neiman and Shapiro 1993; Neiman et al. humidity from the 1000±300-hPa level. The 925-hPa 1993; Blier and Wakimoto 1995). More complete re- level has been added since January 1992. Other surface views can be found in Uccellini (1990), Hoskins (1990), variables include mean sea level pressure (MSLP), sur- and Shapiro and Keyser (1990). face pressure and temperature, temperature and dew- Prior studies have focused on northwestern Atlantic point at 2-m height, and u and y winds at 10-m height events due to their socioeconomical impacts along the (at 2 m since February 1994). North American coast and better data coverage. The The ECMWF operational model is a primitive-equa- Greenland±Iceland region has received much less at- tion-based global spectral model with T213 truncation tention and is traditionally thought to have little cyclo- (roughly 0.558 resolution) and 31 levels (L31) from sur- genetic activity with the exception of polar lows (e.g., face to 10 hPa (Simmons et al. 1995). It was T106/L19 Sardie and Warner 1985; Businger and Reed 1989; Nor- before September 1991 and T63/L16 before May 1985, deng and Rasmussen 1992). More recently, Serreze et with various improvements over the period (Simmons al. (1993) and Serreze (1995) have shown that cyclo- et al. 1989, 1995; Bengtsson 1991). The model employs genesis is common near Iceland during cold seasons, a four-dimensional data assimilation (FDDA) scheme to and Rogers (1997) found that when the storm track is analyze data with multivariate three-dimensional opti- active in extreme northeastern Atlantic, over 20% of the mum interpolation (3D OI) and an injection period of days experience 24-h pressure changes in excess of 20 6 h (Lorenc 1981), and is initialized four times a day hPa. Examples of rapid cyclogenesis in this region in- (0000, 0600, 1200, and 1800 UTC) using nonlinear nor- clude the record-setting case on 14±15 December 1986 mal mode initialization (Wergen 1989). (Gadd et al. 1990), the Braer storm (McCallum and The ECMWF FDDA system uses the 6-h forecast Grahame 1993), the one on 30 September 1995 (L.